I've just finished a clone of Fender Champ. Voltages were fine, so I proceeded to other tests. It sound OK, but there's hum. It's still not in the chassis, since I'm experimenting with that. Since that's my first working tube amp, I really have no much experience, so I need advices and help.
At first I was running it with full wave SS rectifier and AC filament supply. But the hum was really strong, so I tried with DC filaments (used battery with reference to circuit ground for that). Not much improvement.
Then I tried to use high voltage regulated power supply I've build recently (using MOSFET for regulation, I think I've found a schematic for that here, called Elvee PS) for B+ and AC for filaments and hum reduced significantly. Tried the same with battery for filaments, and hum increased a little!
Then I noticed that my guitar's volume pot contributes to hum level. It's smallest when volume is at zero, then it increase dramatically when pot is at the middle point, and then decrease when pot is from 9 to 10. No idea why.
How to troubleshoot this?
At first I was running it with full wave SS rectifier and AC filament supply. But the hum was really strong, so I tried with DC filaments (used battery with reference to circuit ground for that). Not much improvement.
Then I tried to use high voltage regulated power supply I've build recently (using MOSFET for regulation, I think I've found a schematic for that here, called Elvee PS) for B+ and AC for filaments and hum reduced significantly. Tried the same with battery for filaments, and hum increased a little!
Then I noticed that my guitar's volume pot contributes to hum level. It's smallest when volume is at zero, then it increase dramatically when pot is at the middle point, and then decrease when pot is from 9 to 10. No idea why.
How to troubleshoot this?
Hi rankot. I'm currently working on an AC4 project over on the EL34world forum. There's tons of Champ info there and lots of folks who have built them in every conceivable configuration. You might do a search over there, too.
Best,
David
Best,
David
What don you mean by that?
No way to build a Tube amp without a chassis!!!
Please show pictures.
Here are my oscilloscope diagrams for grids of triode 1, triode 2 and pentode, respectively.
It's obvious that some of that hum is present even on triode 1 grid.
It's obvious that some of that hum is present even on triode 1 grid.
Just had a new amp build do the same.
I connected zero volt line to earth and all the noise went away.
A clone of WHICH Fender Champ?
Does your plate supply for the output transformer come from the first filter cap? SIngle ended amps powered like that will be hummy. LOok at the 5F1 Champ for the circuit I describe. Now compare to the 5C1 or 5E1, and note the extra stage of filtration before the main B+ tap.
Alsos, is the heater supply center tapped? And is that grounded? If not, then a pair of 100 ohm resistors for a virtual center tap, a solution Fender has used for decades. In those old Champ models fender just grounded one side of the heater string, but center tap works better. If you don't reference the heaters to ground at all, hum will result.
A chassis acts as shielding and also usually part of the grounding system.
Does your plate supply for the output transformer come from the first filter cap? SIngle ended amps powered like that will be hummy. LOok at the 5F1 Champ for the circuit I describe. Now compare to the 5C1 or 5E1, and note the extra stage of filtration before the main B+ tap.
Alsos, is the heater supply center tapped? And is that grounded? If not, then a pair of 100 ohm resistors for a virtual center tap, a solution Fender has used for decades. In those old Champ models fender just grounded one side of the heater string, but center tap works better. If you don't reference the heaters to ground at all, hum will result.
A chassis acts as shielding and also usually part of the grounding system.
When your guitar is at zero you have the wiper shorted to ground. The pots are 250-500k. In the mid position (linear pot) you would have (In the case of a 500k) 250k to ground, 250k to your pickup, say another 10k, not worth my trouble calculating. So mid point you have 125k resistance across the input. With the volume on full you have 500k in parallel with the pickup impedance, just for kicks let's say 9k.
So both at minimum volume or maximum volume you are shorting out the input, at mid point you are not. That is why you are hearing more hum when not at the end points.
Quick question. Do you have the board shielded and the shield referenced to ground with the board out?
This is what's happening, right here. It's the lack of shielding that lets the 60Hz field into the high-impedance (when around midpoint) pot wiper signal. The chassis is a major part of the shield in such tube circuits.
Great, if that is the case! I will make a chassis ASAP. Now I see that maybe I didn't filter B+ for preamp stage enough, I put 220 Ohm resistor instead of 22k! Will try to change that, too. My schematic and PCB are here for reference. I used Russian tubes 6N2P-EV and 6P1P-EV.
Parts crossed with the red lines are not installed (they are part of DC filament supply, which I didn't use here), and the red dot is the point where I connected virtual ground for filaments (created with two 200 Ohm resistors). Resistors marked with * are not installed, they were intended as a DC filament elevation.
